Bug 1611554: Part 3 - Use jinja templates to generate GeckoChildProcessServices.java and AndroidManifest_overlay.xml; r=geckoview-reviewers,agi

This patch converts `GeckoChildProcessServices.java` into a jinja template.
We also add an overlay generated from a jinja template for `AndroidManifest.xml`
that provides the definitions for content process services.

Note that even though Gradle supports simple substitution of variables in
manifests, I opted not to use that functionality. Since we need the more
powerful template functionality that jinja provides, I felt that having multiple
ways to substitute information into the manifest would be confusing, so we're
using jinja exclusively.

Differential Revision: https://phabricator.services.mozilla.com/D82578